Radio Frequency Identification (RFID) systems are used for identification and/or tracking of equipment, inventory, or living things. RFID systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices called Tags. In RFID systems, the Interrogator communicates to the Tags using modulated radio signals, and the Tags respond with modulated radio signals. Most commonly, this communication utilizes Time-Division Duplex (TDD) or Half Duplex techniques. After transmitting the message to the Tag (called the Downlink), the Interrogator then transmits a Continuous-Wave (CW) radio signal to the Tag. The Tag then modulates the CW signal using modulated backscattering where the antenna is electrically switched, by the modulating signal, from being an absorber of RF radiation to being a reflector of RF radiation. This modulated backscatter allows communications from the Tag back to the Interrogator (called the Uplink).
Prior art Modulated Backscatter (MBS) technology is described in U.S. Pat. Nos. 4,075,632, issued Feb. 21, 1978 to H. A. Baldwin et al. and entitled "Interrogation, And Detection System," and 4,360,810, issued Nov. 23, 1982 to J. A. Landt and entitled "Multichannel Homodyne Receiver". MBS systems typically utilize the amplitude modulated techniques described above for communications from the Interrogator to the Tag, and utilize MBS for communications from the Tag to the Interrogator.
RFID applications exist which have different data rate and range requirements. To accomplish great range, the data rate, especially in the Uplink, must be reduced. This is accomplished by backscatter modulating an unmodulated subcarrier onto a received CW signal for a duration of a few tenths of a second. The Interrogator then listens for an unmodulated tone on the reflected CW signal after the CW signal has undergone homodyne detection.
In order to implement higher bit rates, the Tag would generate an information signal, modulate that information signal upon a subcarrier, and use that modulated subcarrier to backscatter modulate the CW radio signal. In the Interrogator, after detection, the information signal is then demodulated from the subcarrier signal.
Undesirably, such prior art RFID systems are generally "single mode", in the sense that the Tag is capable of operating at either long range mode or higher bit rate mode rather than being capable of operating in both modes.